Social networking services such as, for example, Facebook® and Twitter® are widely used worldwide. Via such social networking services, users freely post descriptions of their activities, their ideas and/or opinions, images and/or videos, news, and/or any other digital content that they want to share with others. Also, the users can express their opinion with regard to other users' posts by simply using the “like” function of Facebook® and/or the “retweet” function of Twitter®.